1. Field of the Invention
The present invention relates to a thin film transistor (TFT) including an oxide semiconductor thin film layer, a method of manufacturing the same, and a display unit including such a thin film transistor.
2. Description of the Related Art
It has been known that an oxide (oxide semiconductor) composed of zinc, indium, gallium, tin, or a mixture thereof shows superior semiconductor characteristics. Thus, in recent years, applying the oxide semiconductor to a TFT as a drive element of an active matrix display has been actively studied.
In the TFT including the oxide semiconductor, electron mobility ten times or more of that of the existing TFT including amorphous silicon is shown and favorable off-characteristics are shown. Thus, the TFT including the oxide semiconductor is largely expected to be applied to a large-screen, high-definition, and high-frame-rate liquid crystal display and an organic EL display.
Meanwhile, in the oxide semiconductor, the heat resistance is not sufficient. Thus, due to heat treatment or plasma treatment in a manufacturing process of the TFT, oxygen is detached and lattice defect is formed. The lattice defect results in forming an electrically shallow impurity level, and causes low resistance of the oxide semiconductor. Thus, in the case where the oxide semiconductor is used for an active layer of the TFT, the defect level is increased, the threshold voltage is decreased, the leakage current is increased, resulting in depression type operation in which a drain current is flown without applying a gate current. If the defect level is sufficiently increased, transistor operation is stopped to shift to semiconductor operation.
Further, in addition to the foregoing lattice defect, hydrogen has been reported as an element to form an electrically shallow impurity level. Thus, in addition to the lattice defect, an element such as hydrogen introduced in manufacturing steps of the TFT has been regarded as a substance that affects characteristics of the TFT including the oxide semiconductor.
Thus, for the purpose of resolving the foregoing disadvantages, for example, TFTs disclosed in “Improved Amorphous In—Ga—Zn-0 TFTs,” Ryo Hayashi et al., SID2008 Proceedings, 2008, pp. 621-624 and Japanese Unexamined Patent Application Publication No. 2007-115808 have been proposed.